Physics Animation Sequence with Transparent Plane
Film tab appears with Cycles Renderer (here v2.79a):


The "Transparent" option in Film tab is for the background only. For any mesh to be transparent as well, you have several options. Make sure you select RGBA in the PNG options in order to include the transparency.


define the material to be transparent:

uncheck the object from rendering:

make it invisible to camera:



I am sure I am missing some other means! Note that all those methods may have side effect like shadows etc.